Three out of four dental patients keep their toothbrushes longer than they should. Whenever you place a toothbrush into your mouth, it can be contaminated with bacteria microbes. These little critters can live on a toothbrush for weeks. It is recommended that you clean your brush regularly and replace it every three months, and always after you have an episode of the flu, cold or other viral infections. Notorious bacteria can implant themselves on the toothbrush bristles leading to re-infection.